We have been examining how sympathetic neurons chose the neurotransmitters that they will use and how target tissues acquire the appropriate complement of receptors and effector proteins. Descriptive studies delineated a developmental change in transmitter phenotype from noradrenergic to cholinergic in the sympathetic innervation of rodent sweat glands. Transplantation and culture experiments indicate that interactions with the target tissue, mediated by a secreted differentiation factor, induce this change. Studies are in progress to identify the sweat gland factor. We identified a second target of cholinergic sympathetic neurons, the periosteum. Analysis of the transmitter properties of the developing periosteal innervation coupled with transplantation studies indicate that these sympathetic neurons, like those that innervate sweat glands, undergo a target-directed transmitter switch. Like the sweat gland factor, the periosteal cholinergic factor is a member of the neuropoietic cytokine family. Sympathetic innervation of sweat glands is required for the glands to undergo a final maturation and become competent to secrete sweat in response to a cholinergic stimulus. Both adrenergic cholinergic stimulation is necessary. Studies using RT-PCR demonstrate that all five muscarinic cholinergic receptor subtypes are present in footpads but only m1 and m3 are required for secretory responsiveness. In the absence of m1 or m3, there is a significant developmental delay in acquisition of normal responsiveness while in the absence of both the glands do not secrete sweat. We are also examining the trophic factor responsiveness of developing sympathetic neurons in vivo. All postmitotic sympathetic neurons require NGF for survival while half also require NT-3. Both NGF and NT-3 activate the tyrosine kinase receptor, trkA. This year we focussed on the role of the low affinity neurotrophin receptor (p75) in mediating NGF and NT-3 actions. Sympathetic neuron number is elevated postnatally in transgenic mice that lack p75 but is normal in adult p75 null mice. Cell culture studies had suggested that p75 can sharpen the specificity of trkA for NGF, decreasing the ability of NT-3 to activate it. Consistent with this notion, we found that mice that were heterozygous for an NT-3 deletion and lacked P75 had normal numbers of sympathetic neurons. Our data suggest that the principal role of the p75 receptor in sympathetic neurons is not to facilitate activation of trkA by NGF or to cause cell death but to sharpen the specificity of trkA for NGF.